This invention relates to current limiter devices for general circuit protection including electrical distribution and motor control applications. In particular, the invention relates to current limiter devices that are capable of limiting the current in a circuit when a high current event or high current condition occurs.
There are numerous devices that are capable of limiting the current in a circuit when a high current condition occurs. One known limiting device includes a filled polymer material that exhibits what is commonly referred to as a PTCR (positive-temperature coefficient of resistance) or PTC effect. U.S. Pat. No. 5,382,938, U.S. Pat. No. 5,313,184, and European Published Patent Application No. 0,640,995 A1 each describes electrical devices relying on PTC behavior. The unique attribute of the PTCR or PTC effect is that at a certain switch temperature the PTCR material undergoes a transformation from a basically conductive material to a basically resistive material. In some of these prior current limiter devices, the PTCR material (typically polyethylene loaded with carbon black) is placed between pressure contact electrodes.
U.S. Pat. No. 5,614,881, to Duggal et al., issued Mar. 25, 1997, the entire contents of which are herein incorporated by reference, discloses a current limiter device. This current limiter device relies on a composite material and an inhomogeneous distribution of resistance structure.
Current limiter devices are used in many applications to protect sensitive components in an electrical circuit from high fault currents. Applications range from low voltage and low current electrical circuits to high voltage and high current electrical distribution systems. An important requirement for many applications is a fast current limiting response time, alternately known as switching time, to minimize the peak fault current that develops.
In operation, current limiter devices are placed in a circuit to be protected. Under normal circuit conditions, the current limiter device is in a highly conducting state. When a high current condition occurs, the PTCR material heats up through resistive heating until the temperature is above the "switch temperature." At this point, the PTCR material resistance changes to a high resistance state and the high current condition current is limited. When the high current condition is cleared, the current limiter device cools down over a time period, which may be a long time period, to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiter device is again capable of switching to the high resistance state in response to future high current condition events.
Known current limiter devices comprise electrodes, electrically conductive composite material, a low pyrolysis or vaporization temperature polymeric binder and electrically conducting filler, combined with an inhomogeneous distribution of resistance structure. The switching action of these current limiter devices occurs when joule heating of the electrically conducting filler in the relatively higher resistance part of the composite material causes sufficient heating to cause pyrolysis or vaporization of the binder. During operation of known current limiter devices, at least one of material ablation and arcing occur at localized switching regions in the inhomogeneous distribution of resistance structure. The ablation and arcing can lead to at least one of high mechanical stresses, which are often in the form of moment mechanical stresses, subjected on the conductive composite material. These high mechanical stresses often lead to the mechanical failure of the composite material.
Therefore, electrically conductive composite materials and their configurations for use in current limiter devices should possess desirable and constant consistent, predictable, or reproducible properties. These properties are suitable for high current multiple use current polymer limiting devices that avoid a build up of undesirable high moment mechanical stresses.